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| United States Patent |
5,290,415
|
|
Shimamune
, et al.
|
March 1, 1994
|
Electrolytic electrode
Abstract
An electrolytic electrode comprising a substrate made of a valve metal, an
intermediate layer formed on a surface of the substrate containing an
oxide of at least one metal selected from the group consisting of niobium,
tantalum, titanium, and zirconium, and a coating layer formed on the
intermediate layer containing an iridium-tantalum mixed oxide and
platinum. In separate embodiments of the invention, the intermediate layer
may contain platinum and/or a stabilizing layer formed on the coating
layer containing an oxide of at least one metal selected from the group
consisting of tin, titanium, tantalum, zirconium, and niobium.
| Inventors:
|
Shimamune; Takayuki (Tokyo, JP);
Nakajima; Yasuo (Tokyo, JP)
|
| Assignee:
|
Permelec Electrode Ltd. (Fujisawa, JP)
|
| Appl. No.:
|
934229 |
| Filed:
|
August 25, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
204/290.09 |
| Intern'l Class: |
C25B 011/08 |
| Field of Search: |
204/290 F
429/40
|
References Cited
U.S. Patent Documents
| 4481097 | Nov., 1984 | Asano et al. | 204/290.
|
| 4517068 | May., 1985 | Hinden et al. | 204/290.
|
Primary Examiner: Gorgos; Kathryn
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
What is claimed is:
1. An electrolytic electrode comprising a substrate made of a valve metal,
an intermediate layer formed on a surface of the substrate containing an
oxide of at least one metal selected from the group consisting of niobium,
tantalum, titanium, and zirconium, and a coating layer formed on the
intermediate layer containing an iridium-tantalum mixed oxide and
platinum, wherein the contents of iridium, tantalum, and platinum in the
coating layer are from 50 to 70 mol %, from 20 to 49.5 mol %, and from 0.5
to 10 mol %, respectively.
2. An electrolytic electrode as claimed in claim 1, wherein the content of
platinum in said coating layer is 2 to 6 mol %.
3. An electrolytic electrode comprising a substrate made of a valve metal,
an intermediate layer formed on a surface of the substrate containing
platinum and an oxide of at least one metal selected from the group
consisting of niobium, tantalum, titanium, and zirconium, and a coating
layer formed on the intermediate layer containing an iridium-tantalum
mixed oxide and platinum, wherein the contents of iridium, tantalum, and
platinum in the coating layer are formed 50 to 70 mol %, from 20 to 49.5
mol %, and from 0.5 to 10 mol %, respectively.
4. An electrolytic electrode as claimed in claim 3, wherein the content of
platinum in said coating layer is 2 to 6 mol %.
5. An electrolytic electrode comprising a substrate made of a valve metal,
an intermediate layer formed on a surface of the substrate containing an
oxide of at least one metal selected from the group consisting of niobium,
tantalum, titanium, and zirconium, a coating layer formed on the
intermediate layer containing an iridium-tantalum mixed oxide and
platinum, and a stabilizing layer formed on the coating layer containing
an oxide of at least one metal selected form the group consisting of tin,
titanium, tantalum, zironcium, and niobium, wherein the contents of
iridium, tantalum, and platinum in the coating layer are from 50 to 70 mol
%, from 20 to 49.5 mol %, and from 0.5 to 10 mol %, respectively.
6. An electrolytic electrode as claimed in claim 5, wherein the content of
platinum in said coating layer is 2 to 6 mol %.
7. An electrolytic electrode comprising a substrate made of a valve metal,
an intermediate layer formed on a surface of the substrate containing
platinum and an oxide of at least one metal selected from the group
consisting of niobium, tantalum, titanium, and zirconium, a coating layer
formed on the intermediate layer containing an iridium-tantalum mixed
oxide and platinum, and a stabilizing layer formed on the coating layer
containing an oxide of at least one metal selected from the group
consisting of tin, titanium, tantalum, zironcium, and niobium, wherein the
contents of iridium, tantalum, and platinum in the coating layer are from
50 to 70 mol %, from 20 to 49.5 mol %, and from 0.5 to 10 mol %,
respectively.
8. An electrolytic electrode as claimed in claim 7, wherein the content of
platinum in said coating layer is 2 to 6 mol %.
Description
FIELD OF THE INVENTION
The present invention relates to an electrode for use in electrolysis
(hereinafter referred to as "electrolytic eletrode") having good
durability. More particularly, this invention relates to an electrolytic
anode containing platinum suitable for use in electrolytic metal plating
or electrolytic surface treatment, from which oxygen is evolved during
electrolysis.
BACKGROUND OF THE INVENTION
Electrolytic plating or electrolytic surface treatment of metals has been
conducted using an article to be treated as the cathode and using, as the
counter electrode, a soluble anode or an insoluble anode comprising a
corrosion-resistant material such as lead or a lead alloy.
Soluble anodes have conventionally been used extensively because,
theoretically, electrolytic plating operations employing a soluble anode
can be conducted continuously without changing the composition of the
electrolyte solution, that is, while the same metal as that deposited on
the cathode (from the electrolyte solution) is released from, and supplied
by, the anode in an amount equal to the deposited amount. However, use of
soluble anodes has generally been defective. For example, the balance
between the anode and cathode is disturbed resulting in the need to
regulate the composition of the electrolyte solution or frequently replace
the anode with a fresh one. Thus, the maintenance of the electrolytic
system is troublesome. Another problem associated with the use of soluble
anodes is that the distance between the cathode and anode is not constant.
Therefore, soluble anodes have been unable to meet the recent demand for
higher quality, higher speed, and energy saving, and as a result,
insoluble anodes which do not dissolve in electrolytic baths (which
changes the composition thereof) and which can be treated independently of
electrodes have come to be used.
Lead or a lead alloy is used as a material for such insoluble anodes. The
lead-based anode has the merits of inexpensiveness and is easily shaped.
However, the use of such lead-based anodes are problematic. For example,
when electrolysis is conducted at a high current density, i.e., at a high
speed, the electrode material dissolves into the electrolyte solution at a
rate of several milligrams per W.H to contaminate the electrolyte
solution, leading to poor product quality. Another problem exists if the
electrolysis is continued further. In such a case, the lead or lead alloy
itself softens resulting in impaired dimensional stability. Although a
platinum-plated titanium electrode is also being used as an insoluble
electrode (in addition to the lead-based electrode), it is expensive, and;
disadvantageously, its life is greatly shortened if on-off operations are
repeatedly conducted.
On the other hand, a so-called dimensionally stable electrode (DSE) was
developed which comprises a valve metal substrate and, provided thereon, a
coating mainly comprising an oxide of a platinum group metal. Use of this
electrode, which is regarded as free from most of the conventional
problems, is spreading rapidly. In particular, in the caustic
soda-producing electrolysis not involving oxygen generation, which is the
mainly employed technique in the present-day industrial electrolysis for
caustic soda production, almost 100% of the electrolytic cells employ a
DSE.
Hitherto, it has been attempted to use the abovedescribed DSE in
electrolysis involving oxygen generation, and the use thereof is rapidly
expanding in recent years. One example of a DSE is an electrode comprising
a substrate made of titanium or a titanium alloy and, formed on the
substrate, an oxide coating containing iridium as an electrode material,
and titanium or tantalum as a stabilizer. The most serious problem
associated with these kinds of electrodes has been that when used in
oxygen-generating electrolysis, the electrode forms a passive-state layer
at the interface between the coating and the substrate and becomes unable
to be used any longer before the electrode material is completely
consumed. As a result of studies by the present inventors, they succeeded
in inhibiting the formation of such a passive-state layer by providing a
thin layer of an electrically conductive oxide at the coating-substrate
interface.
However, even in the case of such an insoluble metal electrode having a
thin layer of an electrically conductive oxide, its life in
oxygen-generating electrolysis is one year at the most when the electrode
is used at a current density of about 100 A/dm.sup.2. This life is
extremely short as compared to the lives of several years or more which
the insoluble metal electrode has when used in soda-producing
electrolysis. Therefore, it has been desired to increase the life of the
insoluble metal electrodes used in oxygen-generating electrolysis.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an insoluble metal
electrode which is mainly used in oxygen-generating electrolysis, and
which can be used over a long period of time while maintaining stable
electrolysis conditions, thereby overcoming the problems associated with
the electrodes known in the art.
In a first embodiment of the present invention, an electrolytic electrode
is provided which comprises a substrate made of a valve metal, an
intermediate layer formed on a surface of the substrate and containing an
oxide of at least one metal selected from the group consisting of niobium,
tantalum, titanium, and zirconium, and a coating layer formed on the
intermediate layer and containing an iridium-tantalum mixed oxide and
platinum.
In a second embodiment of the present invention, an electrolytic electrode
is provided which is the same as the electrode provided in the first
embodiment of the invention except that platinum is added to the
intermediate layer.
In a third embodiment of the present invention, an electrolytic electrode
is provided which is the same as the electrode provided in the first
embodiment of the invention except that a stabilizing layer containing an
oxide of at least one metal selected from the group consisting of tin,
titanium, tantalum, zirconium, and niobium is formed, on the coating
layer.
In a fourth embodiment of the present invention, an electrolytic electrode
is provided which is the same as the electrode provided in the third
embodiment of the invention except that platinum is added to the
intermediate layer.
DETAILED DESCRIPTION OF THE INVENTION
A characteristic feature of the electrode according to the present
invention resides in a small amount of platinum in the coating layer.
In preparing an insoluble metal electrode, it is extremely difficult to
deposit platinum on the surface of an anode in the form of a platinum
oxide which itself is crystalline. Therefore, platinum, in most cases, is
deposited as platinum metal. It is known that, as in the case of
platinum-plated titanium electrodes and similar electrodes, the deposited
platinum metal has considerably inferior corrosion resistance to oxide
coatings such as an iridium oxide coating.
However, as a result of intensive studies by the present inventors, it has
now been found that when iridium containing a minute amount of platinum is
subjected to pyrolysis, an iridium oxide in an extremely good crystalline
state is formed, and the minute amount of platinum is present in the
iridium oxide as a solid solution with the oxide. The present invention
has been completed based on this finding.
In electrodes having an iridium-tantalum mixed oxide coating layer, X-ray
diffraction patterns for the mixed oxide coating layers usually show that
the iridium-tantalum coating layers have a rutile-type crystalline phase
containing an iridium oxide. However, these crystallites are dispersed
because of the poor crystallizability of the oxide and the apparent
crystallite sizes usually are 200 .ANG. or less. It can be easily presumed
that these electrodes, in which the mixed oxide coating layers have such a
crystalline state, have insufficient corrosion resistance and durability
although they have a sufficient activity as an electrode.
According to the present invention, by adding a minute amount of platinum,
which has insufficient corrosion resistance when used alone, into a
coating layer containing iridium and tantalum, the rutile-type crystalline
phase of the iridium and tantalum is further stabilized.
A valve metal is used as the material for the substrate in the electrolytic
electrode of the present invention. Preferred examples of the valve metal
include titanium and titanium alloys. The substrate can be in any suitable
form such as in the form of a net, perforated plate, plate, or rod,
according to the use of the electrolytic electrode to be produced. It is
desirable for the substrate to be activated beforehand by blasting or
acid-washing in order to improve the adhesion between the substrate and an
intermediate layer.
An intermediate layer which contains a semiconducting oxide such as an
oxide of at least one metal selected from the group consisting niobium,
tantalum, titanium, and zirconium is formed on a surface of the substrate.
It is possible to add platinum to the semiconducting oxide. The
semiconducting oxide is not substantially passivated and retains
electrical conductivity even when oxygen generated during electrolysis
migrates to the intermediate layer to form, for example, a rutile-type
stoichiometric oxide. Since the preferred crystalline structure for the
coating layer (described hereinbelow) is of a rutile type, it is desirable
for the intermediate layer to also have a rutile-type crystalline
structure.
In order to ensure electrical conductivity and a rutile-type crystalline
structure for the intermediate layer, preferably, the intermediate layer
is made of an oxide of a mixture containing 50 mol % or more, more
preferably, from 70 to 95 mol % titanium, per 100 mol % of all the
intermediate layer materials, with the remainder being tantalum and/or
niobium. Methods for forming this intermediate layer are not particularly
limited. It is, however, most advantageous to use a thermal decomposition
method from the viewpoints of obtaining an electrically conductive oxide
and ease of operation.
When the insoluble electrode is continuously used over a long period of
time under severe conditions, e.g., in a high-speed continuous
zinc-plating line or similar plating line or electrolytic copper foil
production line in which electrolysis is conducted at an anode current
density of about from 100 to 200 .ANG./dm.sup.2, there are cases where
even the intermediate layer containing an oxide of at least one metal
selected from the group consisting of niobium, tantalum, titanium, and
zirconium cannot sufficiently prevent the migration of oxygen and the
formation of a passive-state layer. In such a case, the formation of a
passive-state layer can be effectively prevented by adding platinum, which
has an oxygen-barrier effect, to the intermediate layer.
It is known that the oxygen overvoltage in sulfuric acid of platinum is
about 300 to 400 mV higher than that of iridium oxide. Therefore, it is
expected that when an iridium oxide-based mixed oxide is used as an
electrode material, no reaction occurs on the surface of the platinum
present with the mixed oxide. However, considerable electrolysis also
takes place on the platinum surface. Because of this, and because platinum
has far lower corrosion resistance than iridium oxide, there is a problem
that if platinum alone is used to constitute an intermediate layer, a
coating layer overlying the intermediate layer peels-off the intermediate
layer.
The present inventors have found that when the content of platinum in the
intermediate layer is 50 mol % or less per 100 mol % of all the
intermediate layer materials, this intermediate layer has advantages in
that the actual oxygen-evolving potential is extremely high, electrolysis
occurs with difficulty on the platinum surface, and the platinum can
exhibit an exceedingly high oxygen-barrier effect, so long as the
intermediate layer is as thin as 1 .mu.m or less.
A coating layer which contains an iridium-tantalum mixed oxide and platinum
is formed on the surface of the intermediate layer. The content of
platinum in the coating layer is generally from 0.5 to 10 mol %,
preferably from 2 to 6 mol %, per 100 mol % of all the coating layer
materials. This is because, as described hereinabove, (1) the effect of
the addition of platinum is sufficiently produced when platinum is added
only in an amount required to allow the platinum to be solid-solubilized
in the crystalline structure of the iridium and tantalum to form a good
crystalline structure, and (2) if the platinum content is too high, the
platinum functions as an electrode material during electrolysis and is apt
to dissolve into the electrolyte solution, leading to breakage of the
coating layer.
The coating layer comprises a mixed oxide of iridium and tantalum, besides
platinum described above. It is desirable for the content of iridium to be
higher than that of tantalum. This is because if the mixed oxide contains
tantalum in an amount larger than that of iridium, the mixed oxide has
difficulty forming a stable rutile-type crystalline structure and use of
an electrolytic electrode having such a coating layer in oxygen-generating
electrolysis results in a slight increase in potential. Since tantalum,
which is added as a stabilizer, is effective in further improving
durability, it is desirable to add tantalum in an amount of at least the
lower limit specified below, in order to improve the stability of the
electrode to be produced. The preferred ranges of the amounts of platinum,
iridium, and tantalum constituting the coating layer are from 0.5 to 10
mol %, from 50 to 70 mol %, and from 20 to 49.5 mol %, respectively, per
100 mol % of all the coating layer materials, respectively.
Methods for forming this coating layer are not particularly limited. It is,
however, desirable if the coating layer is formed by a thermal
decomposition method like the intermediate layer. For example, there is a
method in which a mixture obtained by combining salts of platinum,
iridium, and tantalum, e.g., chloroplatinic acid, iridium chloride, and
tantalum chloride, in a proportion so as to result in a desired
composition, is dissolved in an aqueous solution of hydrogen chloride or
in an organic alcohol solution to obtain a coating liquid. This coating
liquid is coated on the intermediate layer, dried, and the dry coating is
then calcined at a temperature between 450 to 550.degree. C. in air or in
an atmosphere regulated to have an oxygen concentration of about from 15
to 30%. By repeating this procedure, a coating layer having a
predetermined thickness can be obtained. Since the intermediate layer is
made of a semiconducting oxide, too large a thickness of the coating layer
poses the problem of heat generation due to the electrical conductivity of
the coating layer. From the standpoint of avoiding this heat generation
problem and in view of the fact that the coating layer can act as a
substantial oxygen barrier, a smaller thickness is desirable for the
coating layer. Specifically, the preferred range of the thickness of the
coating layer is from 0.1 to 2 .mu.m.
Even the electrode described above, which comprises a valve metal substrate
and, formed thereon, an intermediate layer and a coating layer and which
shows improved corrosion resistance and stability due to such a structure,
is not perfect under all conditions and may have a shorter life according
to use. If a substance which accelerates consumption of the electrode,
e.g., an organic substance, is present in the electrolyte solution, the
life of the electrode is considerably shortened. Therefore, according to
the present invention, the electrode can be made to have far more improved
stability by further providing a stabilizing layer on the surface of the
coating layer.
It is desirable for the stabilizing layer if it is porous to have low
activity as an electrode, and to contain an electrically conductive oxide.
This oxide, in the present invention, is an oxide of at least one metal
selected from the group consisting of tin, titanium, tantalum, zirconium,
and niobium. This oxide can be an oxide of any one of these metals or an
oxide of a combination of two or more of these metals. The stabilizing
layer can be formed by a method similar to the above-described method for
forming the intermediate layer; that is, by coating the coating layer with
an aqueous or alcoholic solution of a metal chloride(s) or metal
alkoxide(s), drying and calcining the coating. Although the mechanism of
the inhibition of electrode consumption due to the stabilizing layer has
not been elucidated, it is presumed that the stabilizing layer serves to
inhibit a corrosive substance in the electrolyte solution from diffusing
into the electrode, thereby preventing electrode consumption.
The degree of the comsumption of the electrode, on which a stabilizing
layer of the above-described kind has been formed, depends to some extent
on the proportion of the metal(s) in the stabilizing layer to the iridium
in the coating layer. In general, the larger the thickness of the
stabilizing layer, the less the electrode is consumed. However, too large
a thickness of the stabilizing layer poses a problem in that due to an
increase in potential, the life of the electrode is shortened.
The present invention will be explained below in more detail by reference
to the following examples which illustrate production methods for
electrolytic electrodes according to the present invention. However, the
electrolytic electrode of the present invention is not limited thereto.
Unless otherwise indicated, all parts percents, ratios and the like are by
weight
EXAMPLE 1
Commercially available titanium plates were sandblasted to roughen the
surface of each plate. These plates were cleaned and then washed in 25 wt
% sulfuric acid at 90.degree. C. for 4 hours to activate the roughened
surfaces. the resulting plate surfaces were coated with a 5% aqueous
hydrogen chloride solution containing titanium chloride and tantalum
chloride in a molar ratio of 20:80, respectively, and the coating was
dried in air and then calcined at 530.degree. C. for 10 minutes. This
procedure was repeated twice, thereby forming an intermediate layer having
a thickness of about 0.5 .mu.m.
Chloroiridic acid was mixed with butyl tantalate in a proportion so as to
result in an iridium:tantalum molar ratio of 6:4, respectively. On the
other hand, 100 parts by volume of butyl alcohol was mixed with 10 parts
by volume of hydrochloric acid. To this liquid mixture, there were added
the above-prepared chloroiridic acid-butyl tantalate mixture and
chloroplatinic acid in various proportions so that the amounts of
platinum, based on the total amount of the iridium and tantalum, were 0,
0.5, 1, 3, 5, 10, and 20% by mol. Thus, coating liquids having various
platinum contents were prepared. Each of the coating liquids were coated
on the intermediate layer formed above, and the coating was dried and then
calcined in air at 530.degree. C. for 10 minutes. This coating and
calcination operation was repeated four times to form a coating layer.
Thus, electrode samples were prepared. Each electrode sample was subjected
to a life test in which electrolysis was conducted at a current density of
100 A/dm.sup.2 in an electrolyte solution prepared by dissolving 200 ppm
of glue in 150 g/dm.sup.3 sulfuric acid. The results obtained are shown in
Table 1.
The results in Table 1 show that electrode life was lengthened by the
addition of platinum to the coating layer, but when the coating layer had
a platinum content of 20%, the electrode life was short, far from being
improved. It can, therefore, be thought that the practical range of the
amount of platinum to be added to the coating layer is up to about 10%.
EXAMPLE 2
Using a coating liquid containing titanium, zirconium, and tantalum in a
molar ratio of 60:20:20, an intermediate layer was formed on the surface
of each of the same substrates as those in Example 1 under the same
conditions as in Example 1. Subsequently, coating liquids each containing
platinum, iridium, and tantalum in which the amount of platinum was 1 mol
% based on the total amount of the three metals and the sum of iridium and
tantalum was 99 mol % based on the total amount of the three metals with
the molar ratio of iridium to tantalum being varied as shown in Table 2,
were separately coated on the surface of the intermediate layer formed
above and the coatings were dried and calcined in the same manner as in
Example 1. For each coating liquid, this procedure was repeated four
times, as in Example 1, thereby forming a coating layer by a thermal
decomposition method. Thus, electrode samples were prepared. Each
electrode sample was subjected to a life test in the same manner as in
Example 1, and the results obtained are shown in Table 2.
For purposes of comparison, an electrode sample was prepared in the same
manner as in the above except that a coating liquid containing iridium,
tantalum, and platinum in a molar ratio of 70:29:1 respectively, was used;
also, a coating layer was formed directly on a substrate without forming
an intermediate layer. The life of this comparative electrode was
similarly tested and the result obtained is shown in Table 2.
The results in Table 2 show that electrode samples having higher
iridium:tantalum ratios and an intermediate layer were very effective in
lengthening electrode life. The comparative electrode sample having no
intermediate layer had ended its life, leaving 90% or more of the
electrode materials (iridium and tantalum).
EXAMPLE 3
Commercially available titanium plates were sandblasted to roughen the
surface of each plate. These plates were cleaned and then washed in 25 wt%
sulfuric acid at 90.degree. C. for 4 hours to activate the roughened
surfaces. A first solution of a 5% aqueous hydrogen chloride solution
containing, dissolved therein, titanium chloride and tantalum chloride in
amounts such that the titanium: tantalum molar ratio was 2:8,
respectively, was mixed, in various proportions, with a second solution of
a 5% aqueous hydrogen chloride solution containing chloroplatinic acid in
an amount such that the molar content of platinum in this second solution
was equal to that of the sum of titanium and tantalum in the first
solution, thereby to prepare coating liquids in which the molar ratio of
the sum of titanium and tantalum to platinum varied as shown in Table 3
(platinum contents being 0, 1, 5, 10, 25, 50, 70, and 90 mol %). The
thus-obtained coating liquids were separately coated on the activated
surfaces of the substrate plates, and the coatings were dried in air and
then calcined at 530.degree. C. for 10 minutes. This procedure was
repeated twice to form intermediate layers.
Each of the resulting substrate plates on which an intermediate layer had
been thus formed was examined for oxygen-evolving potential in 150
g/dm.sup.2 aqueous sulfuric acid solution at a current density of 10
A/dm.sup.2. The results obtained are shown in Table 3.
Subsequently, chloroiridic acid was mixed with butyl tantalate in a
proportion so as to result in an iridium:tantalum molar ratio of 6:4,
respectively. Thereto was added chloroplatinic acid in an amount of 5 mol
% based on the total amount of the iridium and tantalum. The resulting
mixture was dissolved in a mixed solvent of hydrochloric acid and butyl
alcohol, thereby to prepare a coating liquid. This coating liquid was
coated on the surface of each of the intermediate layers formed above, and
the coating was dried and then calcined in air at 530.degree. C. for 10
minutes. This coating and calcination operation was repeated four times to
form a coating layer. Thus, electrode samples were prepared. Each
electrode sample was subjected to a life test in which electrolysis was
conducted in 150 g/dm.sup.3 sulfuric acid having a temperature of
80.degree. C. at a current density of 300 A/dm.sup.2. The results obtained
are shown in Table 3.
The results in Table 3 show that the electrode samples, in which the
intermediate layers had a platinum content higher than 50 mol %, had short
lives. This may be because the intermediate layers themselves were
electrolytically active. Table 3 further shows that even in the case of
the electrode sample in which the platinum content was zero, its life was
not as long as when the platinum content in the intermediate layer was,
for example, 5, 10, 25 and 50 mol %. This may be because the intermediate
layer did not produce a sufficient oxygen-barrier effect.
TABLE 1
______________________________________
Platinum content (%)
Electrode life (hr)
______________________________________
0 160
0.5 320
1 430
3 480
6 460
10 315
20 90
______________________________________
TABLE 2
______________________________________
Ir:Ta ratio Electrode life (hr)
______________________________________
80:14 360
80:19 360
70:29 380
55:44 330
45:54 240
70:29 175
(no intermediate layer)
______________________________________
TABLE 3
______________________________________
Platinum content of
Potential of
intermediate layer
intermediate layer
(mol %) (V, vs NHE) Electrode life (hr)
______________________________________
0 3 or more 450
1 3 or more 460
5 2.8 550
10 2.7 640
25 2.5 800
50 2.2 650
70 1.9 300
90 1.9 250
______________________________________
EXAMPLE 4
Under the same conditions as in Example 3, substrates were covered with an
intermediate layer having a metallic composition such that the
titanium:tantalum molar ratio was 6:4, respectively, and the content of
platinum was 25 mol % based on the total amount of the titanium and
tantalum. On the other hand, iridium chloride was mixed with tantalum
chloride in a proportion so as to result in an iridium: tantalum molar
ratio of 7:3, respectively. Thereto was added chloroplatinic acid in
amounts of 0, 0.5, 1, 3, 5, 10, and 20 mol % based on the total amount of
iridium and tantalum. The resulting mixtures were separately dissolved in
a mixed solvent of hydrochloric acid and butyl alcohol, thereby to prepare
coating liquids. Each of these coating liquids was coated on the surface
of the intermediate layer formed above, and the coating was dried and then
calcined in air at 530.degree. C. for 10 minutes. This coating and
calcination operation was repeated four times to form coating layers.
Thus, electrode samples were prepared. Each electrode sample was subjected
to an accelerated electrolysis test under the same conditions as in
Example 3. The results obtained are shown in Table 4.
The results (in Table 4) show that the electrode sample, in which no
platinum was present in the coating layer, had an insufficient life. This
may be because the coating layer containing no platinum was ineffective in
improving durability. Table 4 further shows that in the case of the
electrode sample in which the platinum content was higher than 10 mol %,
its life was also short, far from being improved. This may be because too
high a platinum content resulted in accelerated consumption of the
platinum.
EXAMPLE 5
Commercially available titanium plates were sandblasted to roughen the
surface of each plate. These plates were cleaned and then washed in 25 wt
% sulfuric acid at 90.degree. C. to activate the roughened surfaces. On
the other hand, titanium chloride, tantalum chloride, and niobium chloride
were dissolved in an aqueous hydrogen chloride solution in amounts so as
to result in a titanium:tantalum: niobium molar ratio of 85:10:5,
respectively, thereby to prepare a coating liquid having a free hydrogen
chloride concentration of 10%. This coating liquid was coated on the
activated surfaces of the substrate plates, and the coating was dried in
air and then calcined at 540.degree. C. for 10 minutes. This procedure was
repeated three times to form an intermediate layer.
Subsequently, chloroplatinic acid, iridium chloride, and tantalum chloride
were dissolved in boiling hydrochloric acid in amounts so as to result in
a platinum:iridium:tantalum molar ratio of 2:68:30, respectively, thereby
to prepare a coating liquid having a free hydrogen chloride concentration
of 10%. The thus-prepared coating liquid was coated on the surface of the
intermediate layer formed above, and the coating was dried and calcined.
This procedure was repeated to form a coating layer.
On the surface of the coating layer of each of the thus-treated substrate
plates, an aqueous solution of tantalum chloride, an alcoholic solution of
tin alkoxide, or an aqueous solution of titanium chloride was coated. The
resulting coatings were calcined at 530.degree. C. for 10 minutes, thereby
forming stabilizing layers having a thickness of about 0.2 .mu.m. Thus,
electrode samples were prepared.
The thus-obtained three electrode samples having a stabilizing layer were
compared with an electrode sample which was the same as these electrode
samples except that it did not have a stabilizing layer. All four samples
were subjected to a life test in which electrolysis was conducted at a
current density of 50 A/dm.sup.2 in a 150 g/dm.sup.3 sulfuric acid bath
containing 5% of acetonitrile and having a temperature of 60.degree. C.
The results obtained are shown in Table 5.
The results (in Table 5) show that electrode life was lengthened greatly by
the formation of a stabilizing layer.
EXAMPLE 6
Electrode samples were prepared in the same manner as in Example 5 except
that platinum was added to the intermediate layer in an amount of 25 mol %
per 100 mol % of all the intermediate layer materials. The thus-obtained
electrode samples were subjected to a life test in the same manner as in
Example 5, and further subjected to an accelerated life test in which
electrolysis was conducted at a current density of 300 A/dm.sup.2 in a 150
g/dm.sup.3 sulfuric acid bath having a temperature of 80.degree. C. The
results obtained are shown in Table 6. As a control, the electrode sample
evaluated in Example 5, which had no stabilizing layer, was likewise
tested for life; the results obtained are shown in the lowermost section
of Table 6.
The results (in Table 6) demonstrate that the intermediate layer, in which
platinum had been added, functioned very effectively under conditions
which might accelerate passivation, although the effect of the addition of
platinum was not so significant when the electrodes were evaluated under
conditions which consumed the surface electrode material as in the
acetonitrile bath. Table 6 further shows that in either case, the
formation of a stabilizing layer was effective in lengthening electrode
life.
TABLE 4
______________________________________
Platinum content of coating layer (mol %)
Electrode life (hr)
______________________________________
0 250
0.5 440
1 540
3 690
5 750
10 600
20 300
______________________________________
TABLE 5
______________________________________
Stabilizing layer
Electrode life (hr)
______________________________________
None 350
Tantalum oxide 630
Tin oxide 550
Titanium oxide 580
______________________________________
TABLE 6
______________________________________
Electrode life
Electrode life
in acetonitrile
in sulfuric acid
Stabilizing layer
electrolysis (hr)
electrolysis (hr)
______________________________________
None 340 800
Tantalum oxide 630 1120
Tin oxide 640 1150
Titanium oxide 590 930
None (note) 350 450
______________________________________
(note)
No platinum in the intermediate layer.
As described above, the electrolytic electrode provided in the first
embodiment of the present invention comprises a substrate made of a valve
metal, an intermediate layer formed on a surface of the substrate and
containing an oxide of at least one metal selected from the group
consisting of niobium, tantalum, titanium, and zirconium, and a coating
layer formed on the intermediate layer and containing an iridium-tantalum
mixed oxide and platinum.
In this electrode, the coating layer contains platinum in addition to the
electrode materials, i.e., iridium oxide and tantalum oxide, and the
platinum has been added to the crystalline structure of iridium and
tantalum to form a solid solution, thereby making the crystalline state
better. Therefore, the presence of platinum in the coating layer serves to
improve the durability and corrosion resistance of the electrode, and;
hence, the life of the electrode can be lengthened considerably. Since
platinum is inferior in electrode activity to either of iridium and
tantalum, the activity of the coating layer in the electrolytic electrode
of the present invention is equal to or slightly lower than that of a
coating layer containing no platinum and consisting of iridium oxide and
tantalum oxide only. However, due to the good crystalline structure of the
coating layer, the electrolytic electrode of the present invention shows
excellent durability and has a far longer life than electrolytic
electrodes having a coating layer containing no platinum.
It is desirable that in the coating layer of the electrolytic electrode of
the present invention, the contents of iridium, tantalum, and platinum are
from 50 to 70 mol %, from 20 to 49.5 mol %, and from 0.5 to 10 mol %,
respectively. This is partly because if the molar content of iridium is
higher than that of tantalum, a stable rutile-type crystalline structure
is formed with difficulty and, at the same time, such a high iridium
content results in a slight increase in potential when the electrode is
used in oxygen-generating electrolysis. When platinum is added to the
coating layer only in an amount required to allow the platinum to be
solid-solubilized in the crystalline structure of the iridium and tantalum
to form a good crystalline structure. The effects of the presently claimed
invention are seen. If the platinum content is too high, the platinum
functions as an electrode material during electrolysis and is apt to
dissolve into the electrolyte solution leading to breakage of the coating
layer.
The electrolytic electrode provided in the second embodiment of the present
invention is the same as the electrode according to the first embodiment
of the invention except that platinum has been added to the intermediate
layer. Although use of the electrolytic electrode according to the first
embodiment of the invention does not pose any problem so long as
electrolysis is conducted under ordinary conditions, the electrode may
suffer peeling of the intermediate layer or formation of a passive-state
layer due to penetration of electrolytically generated oxygen when the
electrode is used under severe conditions as in zinc plating. In this
case, by adding platinum, which has an oxygen-barrier effect, to the
intermediate layer, the penetration of oxygen is inhibited and the peeling
of the intermediate layer and the formation of a passive-state layer are
also prevented. Therefore, an electrolytic electrode having a long life
even under severe use conditions can be provided.
The electrolytic electrode provided in the third embodiment of the present
invention is the same as the . electrode according to the first embodiment
of the invention except that a stabilizing layer containing an oxide of at
least one metal selected from the group consisting of tin, titanium,
tantalum, zirconium, and niobium has been formed on the coating layer.
In some cases, even the electrolytic electrode according to the first
embodiment of the invention, which comprises a substrate and formed
thereon an intermediate layer and a coating layer, has a relatively
shorter life. In such a case, the life of the electrolytic electrode can
be further lengthened to a satisfactory extent by forming a stabilizing
layer of the above-described kind.
The electrolytic electrode provided in the fourth embodiment of the present
invention is the same as the electrode according to the second embodiment
of the invention except that a stabilizing layer containing an oxide of at
least one metal selected from the group consisting of tin, titanium,
tantalum, zirconium, and niobium has been formed on the coating layer.
In some cases, even the electrolytic electrode according to the second
embodiment of the invention, which comprises a substrate and formed
thereon an intermediate layer and a coating layer, has a relatively
shorter life, although the electrode has been improved in durability due
to the platinum added to both the intermediate layer and the coating layer
and can hence still have a lengthened life. In such a case, the life of
the electrolytic electrode can be further lengthened to a more
satisfactory extent by forming a coating layer of the above-described
kind.
While the invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one skilled in the
art that various changes and modifications can be made therein without
departing from the spirit and scope thereof.
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